1. Field of the Invention
The present invention relates to an apparatus and method for forming a functional deposited film on a substrate. More particularly, the present invention relates to an apparatus and method for forming a semiconductor deposited film, representatively, such as an amorphous silicon series semiconductor deposited film (the term xe2x80x9csemiconductor deposited filmxe2x80x9d will be hereinafter referred to as xe2x80x9csemiconductor filmxe2x80x9d for simplification purpose) on a desired substrate, which is suitable for use in a semiconductor device such as a thin film solar cell or the like, by way of plasma chemical vapor deposition (hereinafter referred to as plasma CVD). Such amorphous silicon series semiconductor film includes, for instance, an amorphous silicon semiconductor film, an amorphous silicon-germanium semiconductor film, an amorphous silicon carbide semiconductor film, and a microcrystalline silicon semiconductor film.
2. Related Background Art
It is known that an amorphous silicon semiconductor film having a large area can be formed by way of plasma CVD and therefore, a large area amorphous silicon semiconductor device amorphous can be relatively easily formed in comparison with the case of forming a large area single crystalline or polycrystalline silicon semiconductor device. In fact, amorphous silicon semiconductor films have been often used in the preparation of various semiconductor devices required to have a large area, such as solar cells, photosensitive members (or light receiving members) for copying machines, image sensors for facsimiles, and liquid crystal displays having a thin film transistor. These semiconductor devices are greater than semiconductor devices comprising a crystalline semiconductor such as LSI, CCD, and the like in terms of the length or area as a single device. For instance, in the case of a solar cell, when said solar cell is supposed to have a photoelectric conversion efficiency of 10% in order for a general household to be furnished with a house hold power of about 3 kW, it is necessary to establish a power generation area of about 30 m2 by using a plurality of solar cells. In this case, each solar cell is necessary to have a considerably large area.
Now, in order to form an amorphous silicon film, there is generally adopted a film-forming method wherein a silicon (Si)-containing raw material gas such as SiH4, Si2H6, or the like is introduced into a deposition space in which a substrate on which a film is to be formed is arranged, and said raw material gas is decomposed by supplying a high frequency power thereto to case plasma discharge, whereby forming an amorphous silicon film on said substrate, where an RF power with an oscillation frequency of near 13.56 MHz is generally used as the high frequency power.
Here, recently, plasma CVD using a VHF (very high frequency) power has attracted public attention. For instance, in Amorphous Silicon Technology 1992, pp. 15-16 (Materials Research Society Symposium Proceedings Volume 258), there is described that by changing the discharging frequency from RF of 13.56 MHz to VHF, the film deposition rate can be markedly heightened and a good deposited film can be formed at a high speed.
In order to continuously form an amorphous silicon series semiconductor device, there are known a number of apparatus for continuously forming an amorphous silicon series deposited film on a long substrate (for example, a web substrate) to obtain an amorphous silicon series semiconductor device. As a representative example of such apparatus, there can be mentioned a so-called roll-to-roll type continuous plasma CVD apparatus as disclosed, for instance, in U.S. Pat. No. 4,400,409. The roll-to-roll type continuous plasma CVD apparatus described in this document comprises a plurality of glow-discharging chambers (film-forming chambers in other words) communicated with each other. In said document, there is described that large area semiconductor devices having a desired semiconductor junction can be continuously formed by continuously transporting a long web substrate having a desired width in the longitudinal direction along a route of sequentially passing through the glow-discharging chambers. In the case of using such roll-to-roll type continuous plasma CVD apparatus, desired semiconductor devices can be continuously produced over a long period of time without suspending the operation of the apparatus. Thus, there can be attained high productivity.
Now, a VHF plasma CVD method capable of performing film deposition at a high speed is intended to adopt in a plasma CVD apparatus, it is necessary to take into account such items as will be described below.
The term xe2x80x9cVHF plasma CVD methodxe2x80x9d means a plasma-assisted chemical vapor deposition method using a very high frequency (VHF) power as a plasma discharging power.
Now, in order to generate uniform discharge over a large area, when a discharge electrode in a plate form, which is generally used in the case of using a high frequency lying in an RF (radio-frequency) band region (hereinafter simply referred to as xe2x80x9cRFxe2x80x9d), is used, the impedance is not desirably increased and it is difficult for the electrode to efficiently supply a desired electric power in the case of using a high frequency lying in a very high frequency (VHF) band region (hereinafter simply referred to as xe2x80x9cVHFxe2x80x9d). Therefore, it is necessary to use a bar-like shaped antenna or a radial antenna respectively having a small surface area as the discharge electrode for the VHF power in the plasma discharging chamber (the film-forming chamber) of the plasma CVD apparatus. However, in the case where such electrode is used, it is difficult to attain an uniform plasma intensity distribution over a large area in comparison with the case of using the plate electrode and therefore, it is difficult to continuously deposit a desired semiconductor film uniformly over a large area in the film-forming chamber (the discharging chamber) of the plasma CVD apparatus. In addition, the wavelength of VHF is shorter than that of RF and it is, for instance, 100 cm for 300 MHz as a VHF. In this connection, the wavelength emitted approximates the size of the film-forming chamber in which film deposition is performed, where when standing wave is formed, a varied plasma intensity distribution is more liable to occur in comparison with the case of using RF.
Further, in the case of performing film deposition while continuously moving the substrate as in the case of the foregoing roll-to-roll type continuous plasma CVD apparatus, there is an occasion that plasma generated in a given film-forming chamber (a given discharging chamber) becomes uneven in the direction for the substrate to be transported. Such unevenness of the plasma is rectified by virtue of continuous movement of the substrate and because of this, said unevenness of the plasma does not appear as an unevenness for the thickness distribution of a film deposited. However, in this case, because the film deposition in said film-forming chamber takes place under such plasma condition that is varied in the thickness direction for a film deposited therein due to the unevenness of the plasma in the direction for the substrate to be transported, there is a tendency that a deposited film obtained becomes to have an uneven film property in the thickness direction. Thus, there is an disadvantage in that it is not always ensured to continuously form a desirable deposited film having an uniform film property in the thickness direction.
An principal object of the present invention is to eliminate the foregoing disadvantages in the prior art in the case where a VHF plasma CVD method is adopted and to provide a film-forming apparatus and method in which a VHF plasma CVD method capable of performing film deposition at a high speed can be desirably adopted and which make it possible to efficiently form a large area semiconductor film having a substantially uniform film property in the thickness direction at a high speed even when a bar-like shaped electrode having a small surface area is used as the discharge electrode for the VHF frequency power.
Another object of the present invention is to provide a film-forming apparatus and method in which a VHF plasma CVD method capable of performing film deposition at a high speed can be desirably adopted and which make it possible to afford an uniform plasma intensity distribution over a large area even when a bar-like shaped electrode having a small surface area is used as the discharge electrode for the VHF power, where a semiconductor film having a substantially uniform film property in the thickness direction can be efficiently over the entire surface of a large area substrate at a high speed.
A further object of the present invention is to provide a film-forming apparatus comprising at least a vacuum chamber having a film-forming space provided therein, a high frequency power supply means, and a raw material gas introduction means and in which a non-single crystalline silicon series semiconductor film is formed on a substrate positioned in said film-forming space by introducing a raw material gas into said film-forming space through said raw material gas introduction means and supplying a high frequency power into said film-forming space through said high frequency power supply means, wherein said high frequency power supply means comprises a bar-like shaped electrode electrically connected to a very high frequency (VHF) power source, said high frequency power supplied in said film-forming space comprises a VHF power, said bar-like shaped electrode is arranged such that the longitudinal direction of said bar-like shaped electrode intersects the direction for said substrate to be moved, and the length of said film-forming space relative to the direction for said substrate to be moved is in a range of from {fraction (1/16)} to xc2xd of a wavelength of said VHF power supplied in said film-forming space.
A further object of the present invention is to provide a film-forming method for forming a non-single crystalline silicon series semiconductor film on a substrate positioned in a film-forming space by introducing a raw material gas into said film-forming space and simultaneously supplying a VHF power into said film-forming space, wherein said VHF power is supplied through a bar-like shaped electrode, said bar-like shaped electrode is arranged such that the longitudinal direction of said bar-like shaped electrode intersects the direction for said substrate to be moved, and the length of said film-forming space relative to the direction for said substrate to be moved is in a range of from {fraction (1/16)} to xc2xd of a wavelength of said VHF power supplied in said film-forming space.
In the present invention, the substrate is preferred to comprise a long web substrate. The bar-like shaped electrode is preferred to be arranged such that its longitudinal direction becomes substantially perpendicular to the direction for the substrate to be moved. It is also preferred for the bar-like shaped electrode to be arranged such that it is situated substantially at a center of the direction for the substrate to be moved in the film-forming space.
Further, in a preferred embodiment of the present invention, the vacuum chamber has a multi-chambered structure comprising a plurality of vacuum chambers each having a film-forming space communicated with each other, the substrate comprises a long web substrate which is continuously moved in the longitudinal direction while sequentially passing through said film-forming spaces, and a plurality of non-single crystalline silicon series semiconductor films are continuously formed on said long web substrate which is continuously moved while forming a desired non-single crystalline silicon series semiconductor film on said long web substrate by each film-forming space.
According to the present invention, a high quality large area semiconductor film having a substantially uniform film property in the thickness direction can be efficiently formed over the entire surface of a large area substrate at a high speed. Thus, the present invention is suitable particularly for the production of a solar cell.